Radial compressor diffuser

ABSTRACT

A radial compressor diffuser for a radial compressor stage includes a flow channel which extends radially outwards and which has a cylindrical inlet cross-section on a first radius which extends radially inwards and a cylindrical outlet cross-section which extends radially outwards. A radial diffuser outlet blade is provided in the region of the outlet cross-section in the flow channel. The outlet blade prevents known disadvantages of traditional bladed diffusers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2010/064048, filed Sep. 23, 2010 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2009 043 230.2 DE filed Sep. 28, 2009. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention refers to a radial compressor diffuser for a radial compressor stage, wherein the radial compressor diffuser has radial diffuser exit blading.

BACKGROUND OF INVENTION

Radial compressor stages are used in various constructional fauns of turbocompressors. In this case, the type of flow guiding in the radial compressor stages is different depending upon the field of application. The flow guiding in radial compressors features an impeller, a radial diffuser and a discharge casing as the essential elements. This refers especially to the flow guiding of single-stage radial compressors, of individual stages of geared compressors, or the final stages of multistage single-shaft compressors. The radial diffuser can be both of a bladeless design and also designed as a bladed diffuser. The discharge casing is usually constructed as a volute casing.

SUMMARY OF INVENTION

For achieving the best operating characteristics, the volute casing must be designed and configured so that the static pressure is constant over the impeller circumference or over the circumference of the radial diffuser. In order to achieve this, it is necessary for the flow cross section of the volute to be accurately adapted to the flow data which prevails at the exit of the radial diffuser. Since, however, the flow data along the compressor characteristic line changes during operation, this adaptation is frequently successful to only a very limited extent. For example, the volute is accurately adapted to the flow data only for a defined design point, whereas no specific adaptation to the volute is provided at the other operating points (“off-design operating points”) of the compressor. In other words, in dependence upon the respective operating point, an aerodynamic mismatch of greater or lesser extent exists between the impeller and the diffuser on the one hand and the volute casing on the other hand, which corresponding results in negative effects for the operating behavior of the radial diffuser.

A calculation process for calculating the cross-sectional dimension of a volute casing for achieving a constant static pressure across the impeller circumference or diffuser circumference, as is disclosed in Eckert/Schnell “Axial and Radial Compressors”, Springer Verlag, 1961, p. 417 ff., is described below by way of example. According to this, a volute size parameter C is used as the determinative parameter for the cross-sectional measurement, and is calculated as follows:

${C = \frac{720{\pi \cdot c_{u} \cdot r}}{Q}},$

wherein c_(u)=the tangential component of the flow velocity at the inlet into the volute for the volute design point, r=the radius at the volute inlet, Q=the volumetric flow at the inlet into the volute for the volute design point, π=the circle constant.

From this, after conversion, it follows that:

$C = \frac{360}{{b \cdot \tan}\; \alpha_{c}}$ or ${\alpha_{c} = {\arctan \left( \frac{360}{b \cdot C} \right)}},$

wherein b=the width of the diffuser at the volute inlet.

Therefore, it holds good that for a compressor stage with a given volute, characterized by the volute size parameter C, a constant pressure over the circumference of the impeller or of the diffuser is then accurately achieved if the flow angle assumes the value α_(c) according to the above relationship. The flow angle α which is established at the volute inlet is characterized by the impeller and the further development of the flow in the radial diffuser. This angle is by no means constant but changes along the compressor characteristic line. Therefore, an optimum match between impeller, diffuser and volute is to be seen only at the distinguished operating point at which α=α_(c) applies. At operating points which deviate from this operating point, losses are to be expected on account of a

Moreover, the decided design of the volute is frequently omitted and instead of this the impeller and diffuser are combined with a volute which already exists within the scope of a standardized modular construction system. This often occurs for cost reasons, wherein non-optimized operating characteristics are accepted in favor of the cost situation. The above-described problems occur particularly in the case of bladeless radial diffusers, but also in the case of bladed radial diffusers. Particularly in the case of bladeless radial diffusers, a mismatched volute casing frequently has a particularly negative effect upon the operating behavior of the compressor stage.

In the case of a bladed radial diffuser, the losses linked to mismatched volute casings can be largely avoided. With the aid of a bladed diffuser, efficiency advantages over an unbladed diffuser can be achieved, wherein this, however, is achieved only when the bladed diffuser is positioned as close as possible to the impeller. The inlet ratios r₃/r₁ (r₃ refers in this case to the radius on which the radial extent of the radial compressor blading towards the inside ends, and r₁ refers to the radius on which the inlet cross section of the flow passage or of the impeller exit lies), in the case of bladed diffusers, lie as a rule between r₃/r₁=1.05 and r₃/r₁=1.2 for this reason. Bladed diffusers, however, are not always desirable and have inter alia the disadvantage of a restriction of the usable operating range and create increased compressor noise. In a bladed radial diffuser, vibration excitations can possibly occur as a result of impeller-guide wheel interaction, or more specifically, interactions between diffuser blades and impeller blades ensue, which can lead to a vibration excitation of the highly-loaded impeller. Therefore, in the case of a bladed radial compressor diffuser significant fluctuations in the inflow to the diffuser blades are to be expected on account of wake depressions of the impeller, which, as a result of interaction with the diffuser blades inter alia, also leads to the significant increase of compressor noise indicated above. The disadvantageous effects on account of the impeller-guide wheel interaction are more pronounced the smaller the radii ratio r₃/r₁ is.

It is the object of the invention to create a radial compressor which has improved operating characteristics in the case of a non-optimized discharge casing and which is not afflicted with disadvantages which are known from conventional bladed diffusers.

According to the invention, a radial compressor diffuser is created for a radial compressor stage, having a flow passage, which extends radially outwards, and radially on the inside has a cylindrically encompassing inlet cross section on a first radius and radially on the outside has a cylindrically encompassing exit cross section, and which is designed in such a way that during operation of the radial compressor diffuser a gas flow, which discharges from a radial compressor impeller arranged directly upstream of the radial compressor diffuser and enters the flow passage through the inlet cross section, is decelerated for discharging into a discharge volute casing through the exit cross section. In the region of the exit cross section in the flow passage, provision is made for radial diffuser exit blading which has the effect that the discharge angle of the gas flow, which is pronounced by the radial diffuser exit blading, is virtually unaffected by the operating state of the radial compressor impeller, and that the radial extent of the blading towards the inside ends on a third radius, wherein the ratio of the third radius to the first radius is at least 1.2. Also created according to the invention is a radial compressor stage which features the radial compressor diffuser according to the invention.

One advantage of the radial compressor diffuser according to the invention is that for the outer region of the radial diffuser use is made of guide blading which brings about an almost constant discharge angle α with the value α_(c) even in the case of variation of the volumetric flow along the characteristic line, as a result of which within the entire range of the characteristic line an optimum inflow to the volute is achieved and losses in efficiency and compressor operation are avoided.

According to a development of the invention, the radial diffuser exit blading of the radial compressor diffuser has a multiplicity of guide blades arranged over the circumference, the leading edges of which are arranged in an encompassing manner on the third radius. In this case, the multiplicity of guide blades arranged over the circumference preferably have trailing edges which are arranged in an encompassing manner in a region between the third radius and the radius of the exit cross section.

The radial diffuser exit blading according to the invention has a higher inlet radii ratio r₃/r₁ than is the case with conventional bladed diffusers, which is why the guide blading is placed in a zone in which a comparatively low velocity level prevails (the velocity level in the diffuser is approximately proportional to the reciprocal value of the radial extent). As a result, on the one hand the incidence losses at the inlet of the blading are low, and on the other hand such a low velocity level or Mach number level prevails in the constriction between the stages that even towards high volumetric flows the critical mass flow density is not reached. Consequently, with the blading according to the invention, a restriction of the operating range does not take place, as is the case with conventional bladed diffusers. In addition, with the radii ratio r₃/r₁ 1.2, the wake depressions are largely compensated so that negative effects, which are created as a result of impeller-guide wheel interaction, are avoided. The ratio of the third radius to the first radius is preferably at least 1.35.

The radial diffuser exit blading according to the invention has the effect that the pronounced discharge angle ensures an improved inflow to the spiral collecting chamber and the radial extent of the exit blading ends towards the inside ends on the third radius, wherein the ratio of the third radius to the first radius is at least large enough for the disadvantages known from conventional bladed radial diffusers to be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following text, a preferred embodiment of a radial compressor diffuser according to the invention is explained with reference to the attached schematic drawings. In the drawing:

FIG. 1 shows a schematic sectional view of a radial compressor stage according to an exemplary embodiment of the invention; and

FIG. 2 shows a plan view of the radial diffuser exit blading of the radial compressor stage from FIG. 1.

DETAILED DESCRIPTION OF INVENTION

In FIG. 1, a schematic sectional view of a radial compressor stage 1 according to an exemplary embodiment of invention is shown. The radial compressor stage 1 has a radial compressor impeller 3, a radial compressor diffuser 6, and a discharge volute casing 8. The radial compressor impeller 3 is seated upon a shaft 2 for driving the radial compressor impeller 3. During operation of the radial compressor stage 1, gas enters the radial compressor impeller 3 via an impeller inlet 4 of said radial compressor impeller 3, flows through the radial compressor impeller 3 and, via the impeller exit 5 and via a radial compressor diffuser inlet 9, enters the radial compressor diffuser 6.

The radial compressor diffuser inlet 9 is arranged at a defined radial distance—referred to according to FIG. 1 as the radius 10—from the axis of the shaft 2. The radial compressor diffuser 6 also has a passage 7 and an exit 11 which is arranged on the radius 12 and has a defined width 13. A discharge volute casing 8 with a discharge casing inlet 14 adjoins the diffuser 6. The diffuser 6 also has radial diffuser exit blading 15. The radial compressor exit blading 15 is arranged close to the radial compressor diffuser exit 11 and extends between a radius 16, i.e. the radius at the inlet of the radial diffuser exit blading 15, and the region between the third radius 16 and the radius 12 of the exit cross section 11. In the exemplary embodiment shown in FIG. 1, radial diffuser exit blading 15 is provided in the outer region of the radial compressor diffuser 6, just in front of the entry zone of the discharge volute casing 8. The inlet radii ratio, in this case being the ratio between the radius 16 at the inlet of the radial diffuser exit blading 15 and the radius 10 at the radial diffuser inlet 9, lies above the inlet radii ratio in conventional bladed radial diffusers.

In FIG. 2, a part of the radial diffuser exit blading 15 of the radial compressor stage from FIG. 1 is schematically shown. The blading 15 has a large number of guide blades 17 which extend radially between the radius 22 and the radius 16. In this case, the trailing edges 20 of the guide blades 17 are located in each case on the radius 22 and the leading edges 19 of the guide blades 17 are arranged on the radius 16. Furthermore, the guide blades 17 are inclined in relation to a radial direction so that the ensuing flow velocity vector 21 downstream of the guide blades 17 or of the radial diffuser exit blading 15 has the discharge angle α which in FIG. 2 is designated 18. 

1-5. (canceled)
 6. A radial compressor diffuser for a radial compressor stage, comprising: a flow passage, which extends radially outwards, and radially on the inside has a cylindrically encompassing inlet cross section on a first radius, and radially on the outside has a cylindrically encompassing exit cross section, wherein the flow passage is configured to decelerate a gas flow, which discharges from a radial compressor impeller arranged directly upstream of the radial compressor diffuser and enters the flow passage through the inlet cross section, for discharging into a discharge volute casing through the exit cross section, wherein in the region of the exit cross section provision is made in the flow passage for radial diffuser exit blading which has the effect that the discharge angle of the gas flow, which is pronounced by the radial diffuser exit blading, is virtually unaffected by the operating state of the radial compressor impeller, and that the radial extent of the blading towards the inside ends on a third radius, and wherein the ratio of the third radius to the first radius is at least 1.2.
 7. The radial compressor diffuser as claimed in claim 6, wherein the radial diffuser exit blading has a plurality of guide blades arranged over the circumference, the leading edges of which are arranged in an encompassing manner on the third radius.
 8. The radial compressor diffuser as claimed in claim 7, wherein the plurality of guide blades arranged over the circumference have trailing edges which are arranged in an encompassing manner in a region between the third radius and the radius of the exit cross section.
 9. The radial compressor diffuser as claimed in claim 8, wherein the ratio of the third radius to the first radius is at least 1.35.
 10. A radial compressor stage having a radial compressor diffuser according to claim
 6. 